|  | 4.5 | Layer 3 Design
| 4.5.4 | Why large, scalable LANs need to incorporate routers
Routers can be used to provide IP subnets to add structure to addresses. With bridges and switches, all unknown addresses must be flooded out of every port. With routers, hosts using protocols with network-layer addressing can solve the problem of finding other hosts without flooding. If the destination address is local, the sending host can encapsulate the packet in a data-link header and send a unicast frame directly to the station. The router does not see the frame and, of course, does not need to flood the frame. The sending host might have to use ARP. This would cause a broadcast, but the broadcast is only a local broadcast and is not forwarded by the router. If the destination is not local, then the sending station transmits the packet to the router. The router sends the frame to the destination or to the next hop, based on its routing table. Given this routing functionality, it is clear that large, scalable LANs need to incorporate some routers.
| Threaded Case Study
Washington Project: Addressing

The district office should develop a complete TCP/IP addressing and naming convention scheme for all hosts, servers, and network interconnection devices. The implementation of unauthorized addresses should be prohibited. All computers located on the administrative networks should have static addresses. Curriculum computers should obtain addresses by utilizing Dynamic Host Configuration Protocol (DHCP). DHCP provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them. While the district office should design, implement, and enforce the overall addressing scheme for the network, DHCP should be administered by the local sites within the confines of the address blocks they were assigned.